Accurate field-scale simulations of foam enhanced oil recovery are challenging, due to the sharp transition between gas and foam. Hence, unpredictable numerical and physical behavior is often observed, casting doubt on the validity of the simulation results. In this paper a thorough stability analysis of the foam model is presented, to validate the simulation results and lay a foundation for a tailor-made solver, which can both handle large-scale reservoir simulations and accurately resolve front instabilities. We study the effect of a strongly non-monotonous total mobility function arising from foam models on the stability characteristics of the flow. To this end, we generalize the linear stability analysis of Riaz and Tchelepi (20042007) to nearly discontinuous relative permeability functions, and compare the results with those of highly accurate numerical simulations. In addition, we present a qualitative analysis for the effect of different reservoir and fluid properties on the foam fingering behavior. In particular, we consider the effect of heterogeneity of the reservoir, injection rates, and foam quality. Relative permeability functions play an important role in the onset of fingering behavior of the injected fluid (Riaz and Tchelepi, 2006a). Hence, we can deduce that stability properties are highly dependent on the nonlinearity of the foam transition. The foam-water interface is governed by a very small total mobility ratio, implying a stable front. The transition between gas and foam, however, exhibits a huge total mobility ratio, leading to instabilities in the form of viscous fingering. This implies that there is an unstable pattern behind the front. An indication of this behavior was shown in (Farajzadeh et al., 2016) for a similar foam model, but the authors did not provide a satisfying explanation for the cause of these instabilities. Here we closely study the influence of the foam on instabilities at and behind the front, and are able to predict the flow stability for different foam qualities. We deduce that instabilities are indeed able to grow behind the front, but are later absorbed by the expanding wave. The stability analysis, validated by numerical simulations, provides valuable insights about the important scales and wavelengths of the foam model. In this way we remove any ambiguity regarding the effect of grid resolution on the convergence of the solutions. This makes it possible to design a suitable computational solver that captures all the appropriate scales, while retaining computational efficiency.

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